Conventional methods for producing carbon materials comprising graphenes include various methods such as tape removal method (non-patent documents 1 and 2), solvent extraction method (non-patent document 3), substrate method (non-patent documents 4 and 5), and sublimation method (non-patent documents 6, 7, and 8).
(Tape Peeling Method)
Thin sheet graphite is extracted by applying an adhesive tape to graphite and peeling the tape. When the tape is removed, only thin sheet graphite adheres to a surface of the tape. The graphite adhered to the surface of the tape has at least several dozen layers, is in black ash color, and does not transmit light. By pressing the thin sheet graphite adhered to the adhesive surface of the adhesive tape onto a surface of a silicon wafer with a thermally-oxidized film of an appropriate thickness and then removing the tape, a small fraction of the thin sheet graphite is transferred to the surface of the thermally-oxidized film as graphenes having various numbers of layers. Since relatively large graphenes with good crystallinity can be obtained by this method, the method is suitable for preparing a sample to analyze crystal structures, electronic natures, and the like of graphene. However, productivity is significantly low as a method for producing graphenes industrially.
(Solvent Extraction Method)
Graphite materials such as natural graphite and artificial graphite are basically polycrystalline materials, and in the structures thereof, graphite structures with various numbers of layers exist in connection with many material defects. The solvent extraction method is to extract graphenes in the materials through applying an ultrasonic wave in an organic solvent. Depending on the crystallinity of a graphite material used, various forms of graphene-like thin sheets can be obtained. Most of them are detached or separated thin sheet crystallites. Those in micron order in shape mostly have 20 or more layers, while those having several layers are in submicron order. Therefore, it is less likely to extract a large graphene with a small number of layers.
(Substrate Method)
Substrate method is similar to a method for producing a carbon nanotube, and uses a eutectic reaction in which carbons are made solid-soluble and precipitated in metal by employing a transition metal as a catalyst in a thermal CVD reaction. By subjecting a substrate made of Ni, Fe, or the like to a thermal CVD reaction at around 1000° C., graphenes can be precipitated on a surface of the substrate. At the beginning of development of substrate method, substrates made of Ni were mainly used, but currently substrates made of Cu, which can adsorb carbons onto the surface almost without dissolving carbons thereinto, are often used for the synthesis. After formation of graphene film on a surface of a Cu foil by a substrate method, an acrylic resin is coated thereon and cured to fix graphenes on the back surface of the acrylic resin. Then the Cu foil is dissolved completely in acid to form an acrylic resin film with many graphene fragments fixed thereto. To transfer the graphenes to various base materials such as PET film, the base material is laminated onto the graphenes fixed to the acrylic resin film, a laminated product of acrylic resin film/graphenes/base material is formed by heating and pressing by hot pressing, and then the acrylic resin is removed by acetone or the like. By this method, base materials in which a single-layer and multi-layer graphenes have been transferred can be produced relatively easily on experimental level, but the method is a complicated industrial method because metal as a base material should be dissolved completely to extract synthesized graphenes.
(Sublimation Method)
By heating a single-crystal SiC wafer to around 1600° C., Si atoms are sublimed selectively from a surface of the wafer and C atoms are rearranged to form a graphene layer. Since graphenes can be directly formed on a surface of an insulating substrate, and band gap formation can be achieved by controlling the number of layers and doping, this method is considered to be used as a graphene formation technology in device architecture such as sensors and high-speed switching devices. On the other hand, it is difficult to separate graphenes formed, and thus the method is not suitable for producing a graphene material itself.
However, to put the obtained graphenes into practical use taking advantage of functions of graphenes, it is required to combine graphenes with various materials as needed while preventing aggregation, stack, and adhesion of graphenes, because graphenes have the properties of aggregating, stacking, and adhering to each other or in itself easily due to Van der Waals force. If graphenes are stably stacked in multiple layers, graphite is formed, and advantageous functions of the graphenes are lost.
Therefore, to promote practical use of graphenes, it is required to meet the following requirements in producing carbon materials comprising graphenes: 1) synthesizing self-organized graphenes via space in a three-dimensional structure; 2) preventing aggregation, stack, and adhesion of graphenes each other or in itself as much as possible and putting graphenes in a separated state; 3) combining with various materials while maintaining the structure of graphene, but at present, there remains a variety of problems.
The edge and surface of graphene have characteristic functions that other carbon materials cannot exhibit, for example, a catalytic function to accelerate a chemical reaction in a fuel cell battery reaction, and a catalytic function to reduce active oxygen. To make effective use of such edge or surface of graphene, it is necessary to fix three-dimensionally many very thin graphenes in nanometer order in a space and fulfill the functions sufficiently.